A titanium dioxide pigment is the white pigment excellent in hiding power/tinting strength (i.e. hiding power and tinting strength). Therefore, a titanium dioxide pigment is blended into a coating material, an ink, a resin, or the like for use as colorant. Since a titanium dioxide pigment has high surface catalytic activity, a composition blending it has the low light resistance, the low weather resistance, the severe deterioration with time, the intense yellowness, and the poor yellowing resistance such as the large yellowing degree or the like due to exposure. Accordingly, as a technique of suppressing the catalytic activity of a titanium dioxide pigment, a technique of coating its surface with an inorganic compound of silica, alumina, or the like is known. In addition, as a technique of suppressing yellowing degree of a titanium dioxide pigment, a technique of treating its surface with organosilicon compounds is known.
When a resin composition is blended with a titanium dioxide pigment and is processed and molded at high temperature, surface defects tend to occur which are generally referred to as silver streaks, lacing (foaming), pinholes and the like. It is said that such surface defects are caused by the volatile moisture due to crystallization water and adsorption water which the titanium dioxide particles constituting the titanium dioxide pigment have, and by the volatile moisture due to crystallization water and adsorption water generated from the surface coating layer which is made of an inorganic compound of silica, alumina, or the like and which is treated to suppress the catalytic activity of the titanium dioxide particles for improvement in the light resistance and the weather resistance. In particular, since the desorption of crystallization water from the surface coating layer which is made from an inorganic compound formed by a wet method occurs at high temperature, it is assumed that the desorbed crystallization water forms into volatile moisture to cause the surface defects. Furthermore, the volatile moisture causes the hydrolysis reaction of the resin, resulting in problems that the strength of the resin decreases and the discoloration occurs. Simply, these problems are collectively referred to as “a reduction in the heat resistance of a resin”.
In order to prevent the reduction in the heat resistance of a resin, Patent Literature 1 discloses a method for coating the surfaces of titanium dioxide particles with silica and alumina and then reducing the amount of volatile moisture to a predetermined value or less by firing them. Patent Literature 2 discloses a titanium dioxide pigment having a specific amount of Karl Fischer moisture wherein the surface of the titanium dioxide pigment is formed from the following layers: a coating layer including an aluminum phosphate compound on the surfaces of titanium dioxide particles; and a coating layer including a hydrolysis product of an organosilane compound. Patent Document 3 discloses optically functional particles in which an alkaline earth metal and a condensed phosphate are present on titanium dioxide particles having an average primary particle diameter of 0.001 to 0.1 μm, which are quite different from a pigment. Specifically, Patent Document 3 discloses a material having the following functions and properties: optical functions such as photocatalytic function, ultraviolet absorbing capacity, and transparency; and hydrophilic properties.